Determine LayoutType of Embedded PowerBI report

I figured this out. At load, you can look at the configuration settings, which are stored in the report object.

let report = powerbi.get(document.getElementbyId('embed-container'));
let models = window['powerbi-client'].models;
let isMobile = report.config.settings.layoutType == models.LayoutType.MobilePortrait;

 The variable isMobile is a boolean that shows you if the report is mobile or not.

Caution: If you change the layout to MobilePortrait with the report.updateSettings() function, this will no longer work. You could keep track of that in the document at each toggle.

function toggleMobile() {
    let settings = document.isMobilePortrait ? {layoutType:window['powerbi-client'].models.LayoutType.MobileLandscape} : {layoutType:window['powerbi-client'].models.LayoutType.MobilePortrait};
    report.updateSettings(settings).then(() => {document.isMobilePortrait = !document.isMobilePortrait;});
}